1. Field of the Invention
This invention relates to a new process for the preparation of 0-silylated aliphatic hydroxyl compounds by the reaction of aliphatic hydroxyl compounds with triorgano-chlorosilanes in the presence of phase transfer catalysts and to the use of the resulting 0-silyated compounds as starting materials for the preparation of isocyanates containing ester groups.
2. Description of the Prior Art
Numerous processes for the silylation of hydroxyl compounds are known (see e.g. M. V. Kashutina, L. S. Ioffe, K. A. Tartakovskii, Russ. Chem. Rev. 44, 733 (1955) or M. Ialonde, C. H. Chan, Synthesis 1985, 817 or Houben-Weyl, Methoden der organischen Chemie, Volume VI/Ib, page 739 and Volume 13/5, page 190).
The most commonly used silylating agents are triorganochlorosilanes. The known processes for the preparation of aliphatic 0-silylated compounds by the reaction of aliphatic hydroxyl compounds with triorganochlorosilanes in the liquid phase have the disadvantage that satisfactory yields can only be obtained if the hydrogen chloride formed is bound by bases since the reaction of the starting materials must otherwise be expected to be incomplete or side products are liable to be formed. Separation of the salts produced leads to losses in yields and constitutes an additional and therefore cost increasing procedure. Further, the formation of salts in equimolar quantities is a disadvantage from an ecological point of view.
Silylated amides are also frequently used for the silylation of hydroxyl groups In this case, again, the desilylated amide formed in the process of silylation must be removed from the reaction mixture by an expensive procedure.
Silylation with hexamethyl disilazane also has disadvantages. The preparation of hexamethyl disilazane from trimethylchlorosilane and ammonia is accompanied by the formation of ammonium chloride and secondly, our own experiments have shown that some hydroxyl compounds are impossible to silylate with hexamethyl disilazane.
Silylation by the combined use of trimethylchlorosilane and hexamethyl disilazane is also accompanied by the formation of ammonium chloride in an equimolar quantity.
The process described in DE-OS 35 05 746 for silylation by means of trimethylsilyl cyanide is also disadvantageous because of the release of toxic hydrocyanic acid.
Although 0-silylated hydroxyl compounds, in particular 0-silylated aliphatic polyhydroxyl compounds, are interesting intermediate products for the preparation of polyisocyanates containing ester groups (see e.g. DE-OS 3 634 248), the problems mentioned above have previously prevented their use on a large commercial scale.
It was therefore an object of the present invention to provide a new process for the preparation of 0-silylated aliphatic hydroxyl compounds which would be free from these difficulties and would in particular enable the above mentioned compounds to be prepared in high yields without the corresponding production of large quantities of salts or of toxic decomposition products.
This object has been achieved by the process according to the present invention described below.
The process according to the present invention is based on the surprising finding that when so called phase transfer catalysts are used, aliphatic hydroxyl compounds react with triorganochlorosilanes with the release of gaseous hydrochloric acid until one of the starting components has been used up. This must be regarded as distinctly surprising since in the case of aliphatic hydroxyl compounds it is known that establishment of an equilibrium according to the following equation EQU (CH.sub.3).sub.3 SiCl+R-OH.revreaction.(CH.sub.3).sub.3 SiOR+HCl
is favored by the solubility of hydrogen chloride in the alcohol component which prevents complete silylation. Further, it is known that hydrogen chloride has a tendency to react with the hydroxyl compounds to form undesirable by-products (alkyl chlorides), which again leads to a loss of yield (S. H. Langer, C. Connell, I. Wender, J. Org. Chem. 23, 50 (1958)). In the reaction between phenols and triorganochlorosilanes, these side reactions occur to a much smaller extent but the reaction is still accelerated by the catalysts used according to the present invention.